1. Field of the Invention
The present invention relates to an additive of electrolytic solution for a lithium secondary battery, electrolytic solution for a lithium secondary battery us in the same and a lithium secondary battery.
2. Description of the Related Art
For a lithium secondary battery, an electrolytic solution that is obtained by dissolving an electrolyte salt into organic solvent is used. However, there is a problem that, since such an organic solvent used for electrolytic solution causes reductive decomposition on an electrode surface, cycle characteristics of a secondary battery are deteriorated. In order to suppress such characteristic deterioration of a secondary battery, a lithium secondary battery in which a protective layer for suppressing the reaction of electrolytic solution is formed on an electrode surface is used. Examples of such a protective layer include: water-soluble polymer that coats a negative electrode in advance; a film that is formed by adding a film-forming agent such as vinylene carbonate into electrolytic solution and causing a battery charge/discharge reaction; and the like.
However, such films are known to increase battery resistance. Therefore, in order to improve output of the battery, reduction of such battery resistance has been sought.
Whereas, in order to improve the safety of a lithium secondary battery, adoption of polyelectrolytes has been tried. JP-2008-2184 discloses, in order to improve ion conductivity of a solid secondary battery that uses a solid electrolyte material, the solid secondary battery that uses a boron-containing polymer salt mixed in a polymer material. JP-2002-208310-A discloses an invention that utilizes a boron-containing compound for a solid polymer electrolyte. JP-2009-004258-A discloses an invention that uses a boron-containing salt as an electrolyte salt, in order to suppress expansion during storage at high temperature.
In JP-2008-218404-A and JP-2002-208310-A, the boron-containing compound is mixed in the polymer electrolyte. However, there is a problem that, since the polymer electrolyte has low ion conductivity, resistance of the battery becomes high and output characteristics thereof are deteriorated. Also, since a solid electrolyte interface (SEI) made of the materials described in JP-2009-004258-A has low ion conductivity, its battery resistance is considered to be high.